TROPIC ACTION OF LIGHT ON FREELY MOTILE ORGANISMS 323 



a rise of temperature with constant illumination causes the zoospores to 

 move to the further side of the drop, and this, presumably, because their 

 photic irritability is raised *. In addition, Chromulina Woroniniana shows 

 at 5 C. a negatively, but at 20 C. a positively phototactic response to the 

 same intensity of light 2 , while Strasburger found that a deficiency of 

 oxygen raises the phototactic tone. In the absence of oxygen the zoospores 

 of Algae retain their phototactic irritability so long as they remain capable 

 of movement 3 , but it must be remembered that the exposure to light 

 provides not only the stimulus to movement, but also the energy for it 

 by the agency of photosynthesis. Although no thorough researches have 

 been performed upon the influence of chemical agencies, Elfving 4 has shown 

 that etherization raises the sensitivity of Chlantydomonas pulvisculus, and 

 also its phototactic tone. According to Elfving, chloroform inhibits the 

 phototactic irritability without suspending the power of movement, but 

 these results are not in entire agreement with those of Rothert 5 . It is 

 evident, however, that various combinations of factors may be responsible 

 for the appearance of organisms on the surface at certain times of the day 

 or year, whereas at others they sink to a greater or less depth below it. 



Engelmann found that Englena only responded to an incident ray of 

 light when it fell upon the clear hyaline anterior end, and that it did so before 

 the light reached the eye-spot 6 . It does not follow that a similar localiza- 

 tion of irritability is shown in all cases, while the assumption as to the 

 function of the pigment-spot as an eye 7 is devoid of proof, and is merely 

 based upon the analogy with the pigmented ocelli and eye-spots of lower 

 animals. In fact, many zoospores are phototactic, although they have 

 no pigment-spot. 



When the zoospores are exposed to strong light which has been passed 

 through a prism filled with diluted indian ink, so that the intensity 

 diminishes along a plane at right angles to its direction, they move towards 

 the feebler light and across the incident rays. Diatoms and Desmids 

 behave in the same way, but nevertheless, these observations fail to prove 



1 Strasburger, 1. c., p. 56. Strasburger finds (1. c., p. 52) that a sudden fall of temperature pro- 

 duces a transitory backward movement. 



1 Massart, Bull, de 1'Acad. royale de Belgique, 1891, 3* ser., T. xxn, p. 164. 



3 Celakovsky, Ueber d. Einfluss d. Sauerstoffmangels auf die Bewegung einiger aeroben Organis- 

 men, 1898, pp. u, 28. Reprint from the Bull, de 1'Acad. d. sciences de Boheme. 



4 Elfving, Ueber d. Einwirkung von Aether u. Chloroform auf Pflanzen, 1886, p. 13. Reprint 

 from the Ofversigt af Finska Vetensk. Soc. Forh., Bd. xxvm. 



5 Rothert, Jahrb. f. wiss. Bot., 1903, Bd. XXXIX, p. I. 



6 Engelmann, Pfliiger's Archiv f. Physiologic, 1882, Bd. XXIX, p. 396. 



7 Klebs, Unters. a. d. bot. Inst. zu Tubingen, 1883, Bd. I, p. 263; Overton, Bot. Centralbl., 

 1889, Bd. xxxix, p. 114; Franze, ibid., 1894, Bd. LVil, p. 81 ; Schiitt, Peridineen, 1895, p. 98; 

 Zimmermann, Beihefte z. Bot. Centralbl., 1894, Bd. IV, p. 161 ; Senn, in Engler u. Prantl, Natiirl. 

 Pflanzenfamilien, 1900, i. Th., Abth. i, p. 102 ; Kohl, Carotin, 1902, p. 15. On the structure of the 

 eye-spot cf. also Strasburger, Histologische Beitrage, 1900, Heft vi, p. 193. 



Y 2 



